Method of sorting out defective metallic resistors by sensing the current noise index thereof



United States Patent Office 3,249,222 Patented May 3, 19 66 3,249,222METHOD OF SORTING OUT DEFECTIVE METAL- LIC RESISTORS BY SENSING THECURRENT NOISE INDEX THEREOF 1 John G. Curtis, Bradford, Pa., assignor toCorning Glass Works, Corning, N.Y., a corporation of New York NoDrawing. Filed Sept. 13, 1962, Ser. No. 223,520 1 Claim. (Cl. 209-81)'film which is thereafter patterned by spiralling or the like.

For one example of a suitable film, its characteristics and -method ofapplication, reference is made to US. Patent No. 2,564,706 issued toJohn M. Mochel.

The quality of metallic oxide film or the like resistors is determinednot only by the absolute resistance of the resistor and its closeness tothe indicated nominal value thereof, but also by its temperaturecoeificient of resistance; load life test resistance drifts, that is thevariation of resistance with time and load; power overload capability,and the like. Depending on the ultimate use of the resistors, variousdesired limits of acceptability are placed on the above-notedparameters. In the art, resistors which fall within the desired limitsare termed acceptable or normal and those that fall outside of or exceedsaid limits are termed unacceptable, abnormal, or defective. Inpractice, determination of the temperature coefficient of resistance aswell as the load life test resistance drift of a resistor is a tedious,timeconsuming and costly procedure. In the case of the load life test itis also destructive. It, therefore, becomes impractical to measure theseparameters on a 100 percent basis, and in the art they are in factmeasured only on a sample basis which usually represents a smallpercentage of the total lot.

It has been found that metallic oxide film resistors fall outsideacceptable limits of temperature coefficient of resistance and load lifetest resistance drift when they have certain structural and fabricationdefects. Some of these defects are: film scratches, film porosity,checks or cracks in the non-conductive substrate, chips of thesubstrate, excessive end cap-to-film contact resistance, bridges ofundisturbed film as well as bridges of foreign material across the cutpath or in the current path of a spiralled resistor, and the like. Ithas been discovered that these structural and fabricating defects causethe resistor current noise index to rise substantially proportionatelyto the extent of said defects, that is, the greater number of saiddefects found on the resistor, as well as when such defects are moreserious, the resistor current noise index is higher.

As used herein, resistor current noise is defined as unwanted orundesired resistance fluctuations between the resistor terminals while aconstant flow of current is passed through the conductive portion ofsaid resistor within a frequency band, the index of which isa functionof the value of the intensity of the disturbance expressed in decibelsby the relationship,

noise index=20 log V voltage.

It is an object of this invention to provide a nondestructive'method ofsorting out defective resistors rapidly and reliably.

It is another object of this invention to provide a method for rapidlysorting out resistors having an excessivetemperature coetficient ofresistance on a 100 percent basis.

It is still another object of this invention to provide a method forrapidly sorting out resistors having excessive load life resistancevariations on a 100 percent basis.

Additional objects, features, and advantages of the present inventionwill become apparent, to those skilled in the art, from the followingdetailed description, where, by Way of example, only the preferredembodiments of this invention are described.

According to the invention, resistors which are defective orunacceptable due to an excessive temperature coefficient of resistanceor to excessive load life test resistance variations or drifts can besorted out from a production run of resistors by determining the currentnoise index of such resistors which substantially corresponds to anacceptable limit of temperature coefiicient of resistance and anacceptable limit of load life test resistance variation of such a typeof resistor and thereafter rejecting all resistors of the same typehaving a current noise index higher than the determined value of noiseindex.

A typical example of carrying out the present invention is illustratedby the following description. A 100 piece sample lot of individuallyidentified metallic oxide, spiral cut, film resistors, having a ratingof /2 watt and 150K ohms, said film being of the type described in theheretofore noted Mochel patent, were selected for determining thecurrent noise index limits. The normal performance of this type ofresistor was predetermined to be as follows: A temperature coeflicientof resistance of parts per million per degree centigrade; a 1000 hourload life test resistance variation or drift ranging from approximately+0.2 to +0.6 percent.

The current noise index of each of the resistors in the sample lot wasmeasured. Then the resistances of each of said resistors were measuredat +25 C., 15

-C., 55 C., +6S C., and 150 C. and the temperature coefiicients' werecalculated as related to +25 C. The current noise index was measuredagain. Thereafter, a 1000 hour cycled load life test was conducted andthe resistance of each of said resistors was measured. It was found thatall resistors having temperature coefficients greater than 150 parts permillion per degree centigrade and resistance drifts greater than +0.2 to+0.6 percent had a current noise index exceeding 21.3 decibels.

A production run check of the same type of resistors, rejecting allresistors having a noise index higher than 2l.3 decibels, would resultin substantially rejecting all resistors having a temperaturecoefficient greater than 150 parts per million per degree centigrade anda load life test resistance drift greater than +0.2 to 0.6 percent. Itis readily seen that different types of resistors having differentratings and the like will cause the current noise index limit to varyaccordingly, therefore, a noise index limit must be determined for eachtype of resistor. Furthermore, the current noise index limit will ofcourse vary with the desired limits of temperature coefiicients ofresistance and load life test resistance drifts which are consideredacceptable and normal. This will of course vary with the type andquality of resistor being produced.

Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended thatsuch details be limitations upon i the scope of the invention exceptinsofar as set forth in the following claims.

What is claimed is:

A method of sorting metallic oxide film resistors exceeding acceptablevalues of temperature ooefficient of resistance and load life testresistance drift comprising the steps of providing a supply ofresistors,

selecting maximum acceptable values of temperature coeflicient ofresistance and resistance drift of said resistors,

selecting a sample lot from said supply of resistors,

applying electrical energy to said sample lot of resistors Whilequantitatively sensing the current noise index thereof, quantitativelysensing in any order the resistance and temperature of said sample lotof resistors to obtain the load life test resistance drift, and thetemperature coefiicient of resistance thereof,

4 selecting a maximum acceptable value of current noise index which isequivalent to said maximum acceptable values of temperature coeflicientof resistance and resistance drift, applying electrical energy to theremaining resistors of said supply of resistors while sensing thecurrent noise index thereof, and rejecting those resistors having acurrent noise index exceeding said acceptable value of current noiseindex. 7

References Cited by the Examiner V UNITED STATES PATENTS 1,931,05410/1933 Butterfield 32462 WALTER L. CARLSON, Primary Examiner.

CLAUDE A. S. HAMRICK, EDWARD E. KUBASIE- WICZ, Assistant Examiners.

